It is known that a meandering, or wobbling, groove in an optical disk is used as address information for a data recording track and additional information such as recording conditions. This wobbling groove is used to set a pickup to a desired track position and record data on a track or reproduce data from the track.
On the other hand, recently, illegal copying of contents recorded on optical disks has become a problem, and there has been a desire for robust contents protection systems (CPS: Contents Protection System) for a purpose of protecting the contents.
There is a method of encrypting contents to be stored on a disk as one of contents protection systems (CPS). A constitution has been proposed in which key information applied to the decryption of encrypted contents, other content use control information, and the like are recorded in a state of being superimposed on address information in the above-described groove. Patent Document 1 (Japanese Patent Laid-Open No. 2004-213781), for example, discloses a constitution in which key information is recorded in a groove and used.
However, the key information applied to the decryption of the encrypted contents, the other content use control information, and the like are often secret information, and needs to be prevented from being read easily for leakage prevention. On the other hand, when key data is recorded on a disk, the reproducing device of a user having a legitimate right to the use of contents needs to read key constituent bits correctly. This is because when readout precision is decreased, the correct key information cannot be obtained, and the contents cannot be decrypted.
When the key information applied to the decryption of contents is recorded in the above-described groove, for example, it is necessary to satisfy conflicting needs to avoid easy readout and to realize correct readout.
Requirements for a constitution in which CPS data such as key information as application data in a contents protection system (CPS) is recorded on a disk and used include for example the following requirements.
1. High reliability of a reproduced signal of the CPS data.
2. Difficulty in copying the CPS data to another medium.
3. The cost of manufacturing the CPS data storage medium and a recording and reproducing drive is not greatly increased.
There are such requirements, for example.
Patent Document 1 (Japanese Patent Laid-Open No. 2004-213781) described above discloses a constitution in which key information is recorded in a groove and used. This conventional technique is also devised to satisfy the above requirements. However, in the meantime, techniques for deciphering secret information have progressed. A recording constitution providing a high degree of confidentiality is desired.